Default to available styles on api.mapbox.cn endpoint

[1ec5]

The default day and night styles defined by this library’s MGLStyle extension should account for the different styles available on the .cn endpoint. Specifically, since the Mapbox Navigation Guidance Day/Night styles are currently unavailable on api.mapbox.cn, these methods should return the style URLs for Mapbox Streets Chinese (or Mapbox Light Chinese?) and Mapbox Dark Chinese, respectively.

mapbox-navigation-ios/MapboxNavigation/MGLStyle.swift

Line 7 in 6f13eeb

public class var navigationGuidanceDayStyleURL: URL { get { return URL(string: "mapbox://styles/mapbox/navigation-guidance-day-v2")! } }

mapbox/mapbox-gl-native#12417 would make it easier to check the current API base URL. Until that feature is implemented, the API base URL is static anyways, so we can just consult the Bundle directly.

/cc @m-stephen

[m-stephen]

PR here.
/cc @suntony @chriswu42

[1ec5]

Implemented in #1558.